In order for the human eye to have clear vision of an object at different distances (especially near objects), the effective focal length of the eye's crystalline lens is adjusted to keep an image of the object focused as sharply as possible on the retina. This change in effective focal length is known as “accommodation” and is accomplished by varying the shape of the crystalline lens in the eye. Generally, in the unaccommodated emmetropic eye, the curvature of the lens is such that distant objects are sharply imaged on the retina. In the unaccommodated eye, near objects are not focused sharply on the retina because their images lie behind the retinal surface. In order to visualize a near object clearly, the curvature of the crystalline lens is increased, thereby increasing its refractive power and causing the image of the near object to fall on the retina.
The change in the shape of the crystalline lens is accomplished by the action of certain muscles and structures within the eyeball or the “globe” of the eye. The lens is located in the forward part of the eye immediately behind the pupil. It has the shape of a classical biconvex optical lens, meaning it has a generally circular cross section with two convex refracting surfaces. The lens is located generally on the optical axis of the eye, which is typically the straight line from the center of the cornea to the macula in the retina at the posterior portion of the globe. In the unaccommodated eye, the curvature of the posterior surface of the lens (the surface adjacent to the vitreous body) is somewhat greater than the curvature of the anterior surface.
The lens is closely surrounded by a membranous capsule that serves as an intermediate structure in the support and actuation of the lens. The lens and its capsule are suspended on the optical axis behind the pupil by a circular assembly of radially directed elastic fibers called “zonules.” The zonules are attached at their inner ends to the lens capsule and at their outer ends to the ciliary body and indirectly to the ciliary muscle. The ciliary muscle is a muscular ring of tissue located just within the sclera, the outer supporting structure of the eye.
According to the classical theory of accommodation originating with Helmholtz, the ciliary muscle is relaxed in the unaccommodated eye and therefore assumes its largest diameter. The relatively large diameter of the ciliary muscle in this condition causes a tension on the zonules, which pull radially outward on the lens capsule. This causes the equatorial diameter of the lens to increase slightly and decreases the anterior-posterior dimension of the lens at the optical axis. In other words, the tension on the lens capsule causes the lens to assume a flattened state where the curvature of the anterior surface, and to some extent the posterior surface, is less than it would be in the absence of the tension. In this state, the refractive power of the lens is relatively low, and the eye is focused for clear vision on distant objects.
According to the classical theory, when the eye is intended to be focused on a near object, the ciliary muscle contracts. This contraction causes the ciliary muscle to move forward and inward, thereby relaxing the outward pull of the zonules on the equator of the lens capsule. This reduced zonular tension allows the elastic capsule of the lens to contract, causing an increase in the anterior-posterior dimension of the lens at the optical axis (meaning the lens becomes more spherical). This results in an increase in the optical power of the lens. Because of topographical differences in the thickness of the lens capsule, the central anterior radius of curvature may change more than the central posterior radius of curvature. This is the accommodated condition of the eye, where images of near objects fall sharply on the retina.
Presbyopia is the universal decrease in the amplitude of accommodation, which is typically observed in individuals over forty years of age. In a person having normal vision or “emmetropic” eyes, the ability to focus on near objects is gradually lost. As a result, the individual comes to need glasses for tasks requiring near vision, such as reading.
According to the conventional view, the amplitude of accommodation of the aging eye is decreased because of the loss of elasticity of the lens capsule and/or sclerosis of the lens with age. Consequently, even though the radial tension on the zonules is relaxed by contraction of the ciliary muscle, the lens does not assume a greater curvature. According to this conventional view, it is not possible to restore the accommodative power to the presbyopic eye by any treatment. The loss of elasticity of the lens and its capsule is seen as irreversible. One solution to the problems presented by presbyopia is to use corrective lenses for close work or possibly bifocal lenses if corrective lenses are required for distant vision. Other solutions may include surgically reshaping the cornea of the eye or implanting a presbyopic intra-ocular lens in the eye
Contrary to the conventional view, it is possible to restore the accommodative power to a presbyopic eye by implanting scleral prostheses within the sclera of the eye. For each individual scleral prosthesis, an incision is made in the sclera of the eye, such as near the plane of the equator of the crystalline lens. The incision is then extended under the surface of the sclera to form a scleral “tunnel,” and a scleral prosthesis is placed within the tunnel. A typical scleral prosthesis could be formed from a generally rectangular-shaped bar approximately five millimeters long, one and a half millimeters wide, and one millimeter tall. One or multiple scleral prostheses may be implanted in a patient's eye to partially or completely restore the accommodative power to a presbyopic eye. The same or similar technique can also be used to treat glaucoma, ocular hypertension, elevated intraocular pressure, or other eye disorders. This technique is described more fully in the U.S. patents and patent applications incorporated by reference above.